It's actually a carrier thing. MMS file sizes vary with carriers but most are around 1MB. If you have RCS then the size increases but with today's cameras which produce super large photos and videos, even RCS is not enough.
If you want to send large media from the texting app, then in Gallery select the item to send, tap the share icon and then select Quick Share. That will Give you a number of options but the easiest one is to "copy Link" which you can then paste into your text message. The original sized file will be available to the recipient to download.
Alternatively you can use 3rd party pohot storage services like Amazon and Google to share the file.
Galaxy S21 Ultra 512g(VZW), Galaxy Book (original, VZW), Tab S8 Ultra, Tab S4, Galaxy Watch5Pro(VZW), M8 Smart Monitor